FIG. 1 is a schematic diagram of a conventional Z-source inverter 100 including a Z-impedance portion 110 and a three-phase inverter portion 120 coupled to an alternating current (AC) motor 150. Z-impedance portion 110 comprises an inductor 1110 having an output coupled to the negative terminal of a voltage source (VDC) and a negative terminal of a capacitor 1120 via a node 1115. The input of inductor 1110 is coupled to a negative terminal of a capacitor 1130 and inverter portion 120 via a node 1125 known as a negative DC-link.
Z-impedance portion 110 also includes a power switch 1140 coupled to the positive terminal of VDC and coupled to the input of an inductor 1150 and a positive terminal of capacitor 1130 via a node 1135. The output of inverter 1150 is coupled to the positive terminal of capacitor 1120 and inverter portion 120 via a node 1145 known as a positive DC-link.
Inverter portion 120 includes three branches 1210, 1220, and 1230 coupled to AC motor 150 via nodes 1215, 1225, and 1235, respectively. Each of branches 1210, 1220, and 1230 comprise an upper power switch 1250 coupled in series to a lower power switch 1260 via a respective one of nodes 1215, 1225, and 1235. Each power switch 1250, 1260 includes a diode coupled in parallel with a switch and is capable of conducting current in two directions, and is also capable of stopping voltage in one direction.
Power switches 1250, 1260 control the flow of current within a portion of each respective branch of branches 1210, 1220, and 1230. Power switches 1250, 1260 are typically software controlled switches utilizing high frequency pulse width modulation (PWM) techniques.
During operation, one power switch 1250, 1260 in each of branches 1210, 1220, and 1230 is open and the other power switch 1250, 1260 is closed. In this configuration, closing a power switch 1250, 1260 allows current to flow within a portion of the branch, whereas opening the power switch 1250, 1260 prevents current from flowing within that portion. For example, closing the upper power switch 1250 (and opening lower power switch 1260) of branch 1210 allows current to flow from VDC to terminal I1 via the positive DC-link (i.e., node 1145).
A high frequency PWM technique is typically utilized to control the magnitude, phase angle, and the frequency of power output to AC motor 150. That is, while power switches 1250, 1260 are controlled to operate at a substantially constant switching frequency, the switch duty cycles are modulated to produce three-phase voltages of desired magnitude, phase, and frequency.
AC motor 150 includes three terminals (e.g., terminals I1-I3) coupled to inverter portion 120. Terminal I1 is coupled to node 1215, terminal I2 is coupled to node 1225, and terminal I3 is coupled to node 1235. AC motor 150 is energized with power supplied from inverter 100 and produces a mechanical output based on the supplied power.
When high amounts of power (e.g., current greater than the critical current of inductors 1110 and 1150) are supplied to AC motor 150, Z-source inverter 100 functions in a continuous mode. However, when Z-source inverter 100 is supplying low amounts of power or zero power (e.g., current less than the critical current of inductors 1110 and 1150) to AC motor 150, the voltage stored in capacitors 1120 and 1130 gradually increases because the DC-link current is less than the critical current of inductors 1110, 1150, which results in Z-source inverter 100 operating in a discontinuous mode. Should the voltage stored in capacitors 1120 and 1130 exceed the DC-link voltage rating of Z-source inverter 100, Z-source inverter 100 may become damaged.
One method to prevent capacitors 1120 and 1130 from exceeding the DC-link voltage rating of Z-source inverter 100 is to design Z-source inverter 100 to operate in a continuous mode from minimum to maximum loads by increasing the size of inductors 1110 and 1150. Although this effectively prevents capacitors 1120 and 1130 from exceeding the DC-link voltage rating of Z-source inverter 100, larger inductors increase the size and the cost associated with manufacturing Z-source inverter 100.
Accordingly, it is desirable to provide systems and methods for operating inductors in a Z-source inverter in a continuous current mode to control the voltage at the positive DC-link so that the capacitors do not store excessive amounts of voltage. It is also desirable to provide systems and methods for preventing capacitors in a Z-source inverter from storing excessive amounts of voltage without increasing the size and/or cost of manufacturing the Z-source inverter. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.